This invention relates to crossed-field amplifiers and in particular to a crossed-field amplifier capable of stable operation into variable loads such as exhibited by linear accelerators (LINAC's). Crossed-field amplifiers as known in the prior art will not operate stably into linear accelerators.
An input of a linear accelerator which comprises a resonant cavity through which a pulsed electron beam is passed looks like a short circuit to a pulsed RF input signal drive source during the build up and decay of the RF field in the cavity at the beginning and end of the pulsed RF input signal, respectively. During the remainder of the pulse of RF input, the cavity impedance is constant and preferably matched to the input signal source impedance. It is during the short circuit impedance mismatch to a CDCFA signal source that the CDCFA is most vulnerable to becoming an oscillator because of the power reflected back to the CDCFA output from the linear accelerator cavity through a connecting waveguide. A klystron will operate satisfactorily into such a load. However, a klystron having sufficiently high peak and average power output is substantially heavier and occupies substantially more volume than the CDCFA of this invention which is small and light weight.
In order to have stable operation without self-oscillation when operating into a linear accelerator, an amplifier must have a high degree of RF isolation between its input and its output. Also, to have high gain without self-oscillation, an amplifier must have a high degree of RF isolation between its input and its output. Cathode-driven crossed-field amplifiers (CDCFA) available in the prior art have typically an RF isolation of 30 dB. A typical CDCFA has a frequency band of 14% with an RF gain capability of about 28 dB available before self-oscillation becomes a problem. It is a further object of this invention to increase the RF isolation to as much as 60 to 65 dB in order to obtain more RF gain and also to have the capability of operating into a mismatched load.
In the conventional CDCFA, the RF drive signal is introduced at the source of the electrons. This is accomplished by forming the cathode into a slow-wave structure that will support microwave energy. In the cathode-driven tubes, the amount of RF coupling between the anode and cathode circuits has a strong affect on the tube behavior. In typical forward-wave and backward wave CDCFA's where broadband operation is desired, the cathode and anode circuit diameters have a ratio only slightly greater than one which limits the RF isolation and hence the RF gain. Also, in the typical CDCFA, the cathode structure is tightly coupled to the space charge around the entire interaction space.